The invention relates to a measuring device for measuring magnetic properties of the surroundings of the measuring device with a sensor line with at least two sensor elements extending in a line direction. The invention also relates to a use of such a measuring device.
A possible area of application of the invention is the examination of notes or paper sheets, in particular banknotes, cheques and other paper sheets for the presence of magnetic features, in particular to establish whether the notes or paper sheets have particular, previously fixed magnetic features. Security documents can comprise soft-magnetic security features and/or hard-magnetic security features. Hard-magnetic security features can be produced from materials with high remanence and high coercive field strength. Soft-magnetic security features can be produced from materials with high remanence, but low coercive field strength. Permanent magnets are produced in particular from hard-magnetic materials. Soft-magnetic materials are ferromagnetic materials, which can easily be magnetised in a magnetic field. The associated magnetic polarisation (magnetisation) can be generated for example by a current in a current-carrying coil or by the presence of a permanent magnet. A polarisation thus generated leads in a soft-magnetic material to a magnetic flux density which is many times higher than the externally acting magnetic field generates in the air. Soft-magnetic materials usually have a coercive field strength of less than 1000 A/m. Soft-magnetic materials can however certainly possess saturation magnetisation values comparable with hard-magnetic materials, so that they cannot be distinguished in the saturated state from hard-magnetic materials.
To verify soft-magnetic security features in a security document, therefore, an external magnetic field must be present which saturates the soft-magnetic materials as far as possible. Large magnets are incorporated in measuring devices known from practice, which large magnets generate a sufficiently strong field for the magnetisation of the soft- and hard-magnetic security features, but which also make the structure of such measuring devices complicated. It is known from DE 696 08 137 T2 to provide a reading head, which comprises magnetisation means and at least one magnetoresistive element, wherein the magnetoresistive element has a variable resistance, which is dependent on the magnetic flux applied to it. The reading head described there is used in such a way that each security document is to be identified during the relative movement of the security document with respect to the reading head is first passed in front of the magnetisation means and then in front of said magnetoresistive element, wherein the magnetisation means can be designed in such a way that a constant magnetic flux is present at the sensor element.
A magnetic field emerges from hard-magnetic security features by itself alone and without the presence of an external magnetic field, if the hard-magnetic materials have previously been completely and unequivocally magnetised. This magnetic field continues to be present even over a fairly long time. In the course of time, however, statistical processes have an effect such that the hard-magnetic materials can become demagnetised. For example, banknotes often experience knocks or get creased when they are handled. This can lead to a demagnetisation of the hard-magnetic materials. For the measurement of hard-magnetic security features, it is therefore expedient to imprint a new (unequivocal and durable) magnetisation on the hard-magnetic security feature by means of a premagnetisation magnet. This newly imprinted magnetisation is then able to maintain the hard-magnetic security feature over a longer period, at least over the period of the measurement.